The long term goal of the proposed research is to understand how cells preserve genomic integrity. Genetic instability is characteristic of cancer cells and likely explains how they accumulate multiple genetic alterations that promote tumorigenic growth. This proposal aims to define the components of genomic surveillance systems and understand how they work in a coordinated manner to prevent cancer by inhibiting the cell cycle, promoting DNA repair or initiating apoptosis. Specifically, the DNA damage responsive signaling pathways regulated by the ATM (ataxia-telangiectasia mutated) and ATR (ATM and Rad3 related) kinases will be studied. Three specific aims will be pursued. (1) Identify substrates of the ATM and ATR kinases that mediate their diverse functions. Mass spectrometry will be employed to identify candidate substrates, and genetic analysis in ATM-deficient and ATR-deficient cell lines will be used to confirm phosphorylation by these kinases. (2) Determine how the activity of the ATR protein kinase is regulated. The hypothesis that signaling by ATR is regulated by its access to substrates will be tested using genetic methods. (3) Determine if a recently cloned ATR-interacting protein (ATRIP) functions within a checkpoint signaling pathway. The hypothesis that ATRIP is the human homologue of the S. pombe Rad26 checkpoint protein will be tested using both biochemical and genetic methods. This proposal asks for one year of mentored support for the candidate under the guidance of Dr. Stephen J. Elledge at the Baylor College of Medicine followed by four years of support as an independent biomedical scientist. Expertise in mammalian genetics, mass spectrometry, as well as the management and training of technical personnel will be acquired. The award will facilitate the candidate's transition from a mentored research environment to an independent academic research position focused on basic cancer biology.